Such a vane pump can be used, for example, as booster pump in connection with a pressure exchanger wherein the combination of pressure exchanger and booster pump is used in connection with a reverse osmosis system. When the rotor rotates, pump chambers formed by the housing, the rotor and the vanes decrease and increase their volumes. During the increase phase of the volume of the chambers liquid is sucked into the chambers. During the decreasing phase of the volume of the chambers liquid is pumped out of the chambers.